The present invention relates to a grease composition for rolling bearing. More particularly, the present invention relates to a grease composition for rolling bearing having an improved peeling resistance suitable for rolling bearings mounted in automobile electrical parts or engine auxiliary machinery such as alternator, solenoid clutch for car air conditioner, interpulley, electric fan motor and water pump.
In general, the rotary portion of various power plants of automobile engine such as automobile electrical parts and engine auxiliary machinery, e.g., alternator, solenoid clutch for car air conditioner, interpulley, electric fan motor and water pump is provided with a rolling bearing which is lubricated mostly with a grease.
Due to the spread of FF cars, i.e., front wheel drive cars with the engine in the front which are intended for the reduction of size and weight, and the requirement for the enlargement of room, automobiles have been compelled to reduce the space of engine room. Thus, the reduction of the size and weight of electrical parts and engine auxiliary machinery as mentioned above has been accelerated. In addition, the foregoing various parts have been further required of higher performance and higher output.
However, the reduction of size unavoidably causes an output drop. For example, alternator or solenoid clutch for car air conditioner has been designed to operate at a higher speed to compensate for output loss. At the same time, interpulley has been designed to operate at a higher speed. Further, the requirement for reduction of noise has promoted the design of closed engine room accompanied by the rise in the temperature in the engine room. Thus, the foregoing various parts must withstand high temperatures more than ever.
On the other hand, the grease for use in rolling bearing for automobile has heretofore been given requirements mainly concerning lubricity such as prolonged life of bearing lubrication, little grease leakage, excellent low temperature performance, excellent rust-proofing properties and excellent bearing acoustic properties. However, the foregoing trend towards higher speed operation or higher performance has caused new problems. In some detail, a high load is periodically applied to the bearing surface of a rolling bearing, causing premature peeling on the running surface of the bearing. The development of a grease for preventing such a problem has been under way. As a long-lived grease for high speed rolling bearing intended for the prevention of premature peeling there is disclosed a grease composition comprising as an extender (i.e., a thickening agent) a diurea compound mainly terminated by an aromatic hydrocarbon group in JP-A-5-98280 (The term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d), JP-A-5-194979 and JP-A-5-263091.
As mentioned above, the prior art grease composition has comprised a properly selected extender (i.e., a thickening agent) to exhibit an improved peel resistance. However, there is a limit in the improvement of peel resistance attained by the selection of extender alone. Thus, the prior art grease composition cannot meet the demand for further improvement of peel resistance.
It is therefore an object of the present invention to provide a grease composition which is excellent in peel resistance, not to mention lubricity, as compared with the prior art products.
The foregoing object of the present invention will become more apparent from the following detailed description and examples.
The inventors made extensive studies of the mechanism of occurrence of peeling. As a result, it was considered that peeling occurs due to a synergistic effect of rise in the load caused by resonance of bearing, etc. and bending stress developed by the deformation of outer ring while the prolongation of the life of bearing against peeling by the use of a grease is attributed to a damping effect exerted by a grease film sufficiently retained on the rolling body and bearing surface resulting in the reduction of the vibration level during resonance or the maximum load on the rolling body (see xe2x80x9cNSK Technical Journalxe2x80x9d, No. 656, page 1, 1993). The inventors then made further studies paying their attention to the fact that the effect of preventing peeling can be improved by enhancing the damping effect of grease film. As a result, it was found effective to strengthen the gel structure formed by an extender in order to enhance the grease-forming capacity of a grease film and hence enhance the damping effect against impact load. It was further found that as a means for strengthening the gel structure there is effectively employed a method involving the combined use of an inorganic compound filer and a specific organic zinc compound.
The present invention has been worked out on the basis of the foregoing knowledge. In other words, the foregoing object of the present invention is accomplished with a grease composition for rolling bearing comprising a base oil, an extender (i.e., a thickening agent), an inorganic compound-based filler and zinc dithiocarbamate wherein said inorganic compound-based filler has an average particle diameter of not more than 2 xcexcm and comprises at least one material selected from the group consisting of particulate materials made of metal oxides, metal hydroxides, metal carbonates, hydrates thereof, metal nitrides, metal carbides, (synthetic) clay minerals, diamond, and solid lubricants.